1. Field of the Invention
The present invention pertains to the locating of difficult access points, on a topological map plotted on the basis of a map of curvilinear distances.
2. Description of the Related Art
When dealing with a map of the zone overflown by an aircraft, plotted on the basis of a map of curvilinear distances taking account of the vertical flight profile of the aircraft, the difficult access points, which are those whose curvilinear distances greatly exceed the Euclidean distances, correspond to relief zones that are dangerous for the aircraft, the description dangerous applying to any relief zone that cannot be crossed directly by the aircraft starting from its current position having regard to its turning and climbing performance.
The applicant has already proposed, in a French patent application filed on Sep. 26, 2003, under no. 0311320, a method of estimating, on a map extracted from a terrain elevation database, curvilinear distances separating the points of the map, from a reference point taken as origin of the distances having regard to obstacles to be detoured around, the contours of which may change in the course of the time of traversal of the curvilinear distances as is the case for an aircraft whose current position corresponds to that of the point taken as origin of the measurements of the distances and which has to comply with a vertical flight profile with variations in altitude implying that one and the same relief that is threatening at a certain moment is no longer so at another or vice versa. This method implements a propagation-based distance transform also known by the name of chamfer mask distance transform since it uses a so-called “chamfer mask” array cataloging the approximate values of the Euclidean distances separating a point of the map from its nearest neighbors.
The array formed by the curvilinear distances estimated for the set of points of a map is called, for convenience, a map of curvilinear distances. It is not particularly intended to be displayed but rather to serve in the plotting of maps to be displayed showing certain specifics of the relief.
In the case of an aircraft, the map of curvilinear distances relates to the region overflown and has, as reference point taken as origin of the measurements of the curvilinear distances, a point near the current position of the aircraft. It serves for the plotting of a map, often in two dimensions, which is displayed on the instrument panel and shows, in false colors, a split of the region overflown into zones delimited as a function of the capacity of the aircraft to cross them and of the time that the latter would take to reach them when they are crossable, for example red for uncrossable reliefs, no route being possible, yellow for reliefs that are far away or close in the sense of the Euclidean distance but are only crossable by a diverted route and green for reliefs that are close in the sense of the Euclidean distance, and are crossable by a direct route.
A map of the relief overflown, established on the basis of a map of curvilinear distances has the drawback of not giving very explicit information on the importance of the diversion to be accomplished when it is necessary to make one, thereby prompting us to understate, through caution, the zones represented in yellow in favor of those represented in red.
It is possible to obtain this information on the importance of the diversion to be accomplished, on the basis of the calculation of the Euclidean distances and of their comparisons with the curvilinear distances but account has to be taken in these comparisons of the presence of the obstacles to be detoured around and this leads to a considerable increase in the calculations required for the plotting of the map displayed.